Modern integrated circuits are made up of literally millions of active and/or passive devices such as transistors and capacitors. These devices are initially isolated from each other, but are later interconnected together to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnections are increasingly determining the limits of performance and the density of modern integrated circuits. On top of the interconnect structures, bond pads are formed and exposed on the surface of the respective chip. Electrical connections are made through bond pads to connect the chip to a package substrate or another die. Bond pads can be used for wire bonding or flip-chip bonding. In a typical bumping process, interconnect structures are formed on metallization layers, followed by the formation of under-bump metallurgy (UBM) and solder balls. Flip-chip packaging utilizes bumps to establish electrical contact between a chip's I/O pads and the substrate or lead frame of the package. Structurally, a bump actually contains the bump itself and a so-called under bump metallurgy (UBM) located between the bump and an I/O pad. An UBM generally contains an adhesion layer, a barrier layer and a wetting layer, arranged in that order, on the I/O pad. The bumps themselves, based on the material used, are classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals.
Usually, a material used for the solder alloy is so-called Sn—Pb eutectic solder of Sn-38 mass % Pb. Recently the semiconductor industry has been moving to “lead (Pb) free” packaging and lead-free device connector technology. This trend increasingly results in the use of lead free solder bumps and lead free solder balls to form connections with integrated circuits and packages. The use of lead free solder is safer for the environment, safer for workers in the industry and safer for consumers than lead based solder bumps or solder balls. However, the quality and reliability of the solder bumps has not always been as great as desired. For finer pitches and larger interconnect densities, the risk of shorts occurring between solder bumps during fabrication and flip-chip assembly is high.